Known disposable syringes are in many cases constructed so that they can be reused, despite that they are intended only for a single use. One can refill the syringe by a pull of a traditional plunger, which through its connection with a piston refills a cylindrical barrel for secondary injection. The construction of a syringe without any measures to prevent reuse is typically simpler since it does not require such additional measures. In many cases the possibility to reuse a disposable syringe is of no concern. However, due to lack of necessary medical equipment, it has become common in especially poor developing countries to reuse disposable syringes, which only have been scanty cleaned and sterilised due to inadequate sanitary and hygienic conditions.
Examples of disposable syringes are provided in U.S. Pat. No. 5,795,337, WO 2004/078243, WO 2004/075958, WO 2004/033018, U.S. Pat. No. 4,252,118, U.S. Pat. No. 5,158,549, and WO 2002/32485. Further examples of syringes are provided in U.S. Pat. No. 4,430,082, EP 0047442, FR 1600637 and U.S. Pat. No. 3,545,607.
There is however a certain area within injections where the user will go to great lengths to manipulate a disposable syringe for reuse. In connection with vaccination campaigns carried out in poor third world countries, it is rather normal to apply pressure through the needle, which pressure is often created manually by use of a larger standard syringe, which pressure refills the syringe with vaccine through the glass vial for a secondary injection.
Undesired reutilisation is furthermore very widespread among drug abusers where one or more persons unhesitatingly utilise the same syringe and needle without prior sterilisation.
Such reuse of disposable syringes often results in human beings being infected with life-threatening diseases both in hospitals, clinics and vaccination sites all over the world and contribute to the spreading of blood borne, infectious diseases such as HIV virus (AIDS) and hepatitis B, among others.
In recognition of these problems the World Health Organization (WHO) has utilised and urged utilisation of auto-disable (AD) syringes, which have been made impossible to reuse for different structural reasons.
Such AD-syringes are often expensive to manufacture and therefore expensive to use, and many nations cannot afford to buy these kinds of disposable syringes to a necessary extent.
There exists a number of different AD-syringes which are used in connection with vaccination campaigns in the world's poorest countries. Besides containing individual auto disabling techniques, which most often require extra components compared to a normal or traditional syringe, some are also constructed to resist pressure filling, thereby preventing different kinds of attempts to reutilise the syringe.
From PCT/EP00/00028 an AD-syringe is known, which functions by disconnection of the piston from the plunger during emptying of the syringe, since the piston during the emptying sequence turns approximately 90 degrees and thus away from its interconnection with the plunger. The piston is then positioned at the bottom of the barrel and maintained there. The sealing surfaces of the piston have a significant vertical extension in the longitudinal axis of the barrel. The vertical extension of the piston's abutting surface causes the friction to be relatively low regardless whether the piston is in motion or not. The lubrication necessary for the utilisation of the syringe will thus, due to the large vertical abutting surface of the piston, be effective between piston and barrel inner wall regardless of motion or standstill. To resist pressure filling this approach is questionable since the necessary friction in order to resist pressure filling may not be achievable when the lubrication is permanently present in the abutting interface between piston and barrel inner wall.
Like the piston mentioned in the aforementioned patent application, pistons known from traditional syringes as well as AD-syringes are provided with one or more sealing surfaces to ensure tightness between piston and barrel inner wall which pistons are relatively permissive and which means that these, when mounted in the barrel with a diameter smaller than the piston's outside diameter, result in that the sealing surfaces' abutting surfaces are expanded significantly in the vertical direction and thus in the longitudinal axis of the barrel. The permanent presence of the lubrication in the area between the piston and the barrel inner wall means that with a given pressure through the needle one is able to impact and thereby position the piston to the starting position, whereby one will be able to reuse the syringe for a new injection.
Other AD-syringes are provided with a metal clip or one-way valve which efficiently shuts off pressure filling through the needle. Known AD-syringes thus alone avoid pressure filling as a result of the presence of an extra component.
This is because pistons in AD-syringes are predominantly unchanged compared to pistons in traditional syringes and therefore not themselves able to resist pressure refilling of the syringe which is a legal requirement for the AD-syringes that WHO wishes to utilise.
Manufacturing and assembly of the above described AD-syringes present a number of challenges. First, it is desirable to manufacture the syringes with as few components as possible to ensure a low manufacturing price and thereby a wide market in third world countries. Further, the syringes should comprise measures for preventing reutilisation both by regular filling or pressure filling of the syringe. For example, syringes with a releasable connection between piston and plunger, will not be prevented from reuse by pressure filling. Moreover, the previously mentioned solutions to prevent reuse are not suited for implementing in other types of AD-syringes, resulting in a wide variety of different solutions, none of them being co-operable. Finally, the AD-syringes should provide easy usability.
In view of the above the object of the present invention is providing an injector of the kind mentioned in the opening paragraph, that has a simple structure, is easy to operate for one time use, is inexpensive to manufacture, and cannot be reused.